It was investigated whether morpholinosydnonimine (SIN-1), which spontaneously decomposes into NO and 3-morpholinoiminoacetonitrile (SIN-1C), can be used for protection of cardiomyocytes against reoxygenation-induced hypercontracture. Isolated ventricular cardiomyocytes (from adult rats) were used as the experimental model. SIN-1 [concentration with half-maximal effect (EC50) 2.5 x 10(-4) M] and SIN-1C (EC50 8.3 x 10(-3) M) inhibited the contractile response of electrically paced cardiomyocytes. When the cells were submitted to substrate-free anoxia (135 min) and subsequent reoxygenation (30 min), the onset of reoxygenation provoked their hypercontracture. It was studied whether the temporary presence of the test agents during the last 15 min of anoxia and the first 15 min of reoxygenation prevented hypercontracture. At 10 nM, SIN-1 prevented hypercontracture in 96% of the cells and SIN-1C in 72% of the cells. The protective effect of SIN-1 was reduced to that of SIN-1C by simultaneous presence of methylene blue (50 microM). Methylene blue had no influence on the protective action of SIN-1C. SIN-1C (10 mM) plus sodium nitroprusside (another NO donor, 250 microM) provided the same degree of protection as SIN-1 (10 mM). The results show that reoxygenation-induced hypercontracture can be prevented or attenuated by the temporary presence of high concentrations of SIN-1 or SIN-1C. SIN-1 acts through a dual mechanism, protecting through the generation of NO and SIN-1C.